1
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Le-Nguyen D, Chiche L, Hoh F, Martin-Eauclaire MF, Dumas C, Nishi Y, Kobayashi Y, Aumelas A. Role of Asn2 and Glu7 residues in the oxidative folding and on the conformation of theN-terminal loop of apamin. Biopolymers 2007; 86:447-62. [PMID: 17486576 DOI: 10.1002/bip.20755] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
The X-ray structure of [N-acetyl]-apamin has been solved at 0.95 A resolution. It consists of an 1-7 N-terminal loop stabilized by an Asn-beta-turn motif (2-5 residues) and a helical structure spanning the 9-18 residues tightly linked together by two disulfide bonds. However, neither this accurate X-ray nor the available solution structures allowed us to rationally explain the unusual downfield shifts observed for the Asn(2) and Glu(7) amide signals upon Glu(7) carboxylic group ionization. Thus, apamin and its [N-acetyl], [Glu(7)Gln], [Glu(7)Asp], and [Asn(2)Abu] analogues and submitted to NMR structural studies as a function of pH. We first demonstrated that the Glu(7) carboxylate group is responsible for the large downfield shifts of the Asn(2) and Glu(7) amide signals. Then, molecular dynamics (MD) simulations suggested unexpected interactions between the carboxylate group and the Asn(2) and Glu(7) amide protons as well as the N-terminal alpha-amino group, through subtle conformational changes that do not alter the global fold of apamin. In addition, a structural study of the [Asn(2)Abu] analogue, revealed an essential role of Asn(2) in the beta-turn stability and the cis/trans isomerization of the Ala(5)-Pro(6) amide bond. Interestingly, this proline isomerization was shown to also depend on the ionization state of the Glu(7) carboxyl group. However, neither destabilization of the beta-turn nor proline isomerization drastically altered the helical structure that contains the residues essential for binding. Altogether, the Asn(2) and Glu(7) residues appeared essential for the N-terminal loop conformation and thus for the selective formation of the native disulfide bonds but not for the activity.
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Affiliation(s)
- Dung Le-Nguyen
- Centre de Pharmacologie et Biotechnologie pour la Santé-CNRS FRE 3009, Faculté de Pharmacie, Montpellier, France
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2
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Stocker M. Ca2+-activated K+ channels: molecular determinants and function of the SK family. Nat Rev Neurosci 2004; 5:758-70. [PMID: 15378036 DOI: 10.1038/nrn1516] [Citation(s) in RCA: 397] [Impact Index Per Article: 19.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
Ca(2+)-activated K(+) (K(Ca)) channels of small (SK) and intermediate (IK) conductance are present in a wide range of excitable and non-excitable cells. On activation by low concentrations of Ca(2+), they open, which results in hyperpolarization of the membrane potential and changes in cellular excitability. K(Ca)-channel activation also counteracts further increases in intracellular Ca(2+), thereby regulating the concentration of this ubiquitous intracellular messenger in space and time. K(Ca) channels have various functions, including the regulation of neuronal firing properties, blood flow and cell proliferation. The cloning of SK and IK channels has prompted investigations into their gating, pharmacology and organization into calcium-signalling domains, and has provided a framework that can be used to correlate molecularly identified K(Ca) channels with their native currents.
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Affiliation(s)
- Martin Stocker
- Wellcome Laboratory for Molecular Pharmacology, Department of Pharmacology, University College London, Gower Street, London WC1E 6BT, UK.
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3
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Xu CQ, He LL, Brône B, Martin-Eauclaire MF, Van Kerkhove E, Zhou Z, Chi CW. A novel scorpion toxin blocking small conductance Ca2+ activated K+ channel. Toxicon 2004; 43:961-71. [PMID: 15208029 DOI: 10.1016/j.toxicon.2004.01.018] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/19/2003] [Accepted: 01/08/2004] [Indexed: 11/26/2022]
Abstract
Small conductance calcium activated potassium channels (SK) are crucial in the regulation of cell firing frequency in the nervous system and other tissues. In the present work, a novel SK channel blocker, designated BmSKTx1, was purified from the scorpion Buthus martensi Karsh venom. The sequence of the N-terminal 22 amino acid residues was determined by Edman degradation. Using this sequence information, the full-length cDNA and genomic gene of BmSKTx1 were cloned and sequenced. By these analyses, BmSKTx1 was found to be a peptide composed of 31 amino acid residues with three disulfide bonds. It shared little sequence homology with other known scorpion alpha-KTxs but showed close relationship with SK channel blockers in the phylogenetic tree. According to the previous nomenclature, BmSKTx1 was classified as alpha-KTx14.1. We examined the effects of BmSKTx1 on different ion channels of rat adrenal chromaffin cells (RACC) and locust dorsal unpaired median (DUM) neurons. BmSKTx1 selectively inhibited apamin-sensitive SK currents in RACC with Kd of 0.72 microM and Hill coefficient of 2.2. And it had no effect on Na+, Ca2+, Kv, and BK currents in DUM neuron, indicating that BmSKTx1 was a selective SK toxin.
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Affiliation(s)
- Chen-Qi Xu
- Institute of Neuroscience, Chinese Academy of Sciences, 320 Yue-Yang Road, Shanghai 200031, China
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4
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Stocker M, Hirzel K, D'hoedt D, Pedarzani P. Matching molecules to function: neuronal Ca2+-activated K+ channels and afterhyperpolarizations. Toxicon 2004; 43:933-49. [PMID: 15208027 DOI: 10.1016/j.toxicon.2003.12.009] [Citation(s) in RCA: 63] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/03/2003] [Accepted: 12/06/2003] [Indexed: 11/21/2022]
Abstract
Potassium channels regulate the membrane excitability of neurons, play a major role in shaping action potentials, determining firing patterns and regulating neurotransmitter release, and thus significantly contribute to neuronal signal encoding and integration. This review focuses on the molecular and cellular basis for the specific function of small-conductance calcium-activated potassium channels (SK channels) in the nervous system. SK channels are activated by an intracellular increase of free calcium during action potentials. They mediate currents that modulate the firing frequency of neurons. Three SK channel subunits have been cloned and form channels, which are voltage-insensitive, activated by submicromolar intracellular calcium concentrations, and are blocked, with different affinities, by a number of toxins and organic compounds. Different neurons in the central and peripheral nervous system express distinct subsets of SK channel subunits. Recent progress has been made in relating cloned SK channels to their native counterparts. These findings argue in favour of regulatory mechanisms conferring to native SK channels with specific subunit compositions distinct and specific functional profiles in different neurons.
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Affiliation(s)
- Martin Stocker
- Wellcome Laboratory for Molecular Pharmacology, Department of Pharmacology, University College London, Gower Street, London WC1E 6BT, UK.
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5
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Utkin YN. Photoactivatable derivatives of peptide and protein ligands in the study of neuroreceptor structure. RUSSIAN JOURNAL OF BIOORGANIC CHEMISTRY 2000. [DOI: 10.1007/bf02758856] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
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6
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Wadsworth JD, Doorty KB, Ganellin CR, Strong PN. Photolabile derivatives of 125I-apamin: defining the structural criteria required for labeling high and low molecular mass polypeptides associated with small conductance Ca(2+)-activated K+ channels. Biochemistry 1996; 35:7917-27. [PMID: 8672494 DOI: 10.1021/bi9602371] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
The structure of apamin-sensitive Ca(2+)-activated K+ channels has been investigated using high-affinity, photolabile azidoaryl derivatives of 125I-[alpha-formyl-Cys1]apamin and 125I-[epsilon-formyl-Lys4]-apamin. Labeling patterns suggest that similar structural constraints are required for labeling analogous polypeptides associated with distinct channel subtypes. When photoprobes are coupled at the epsilon-amino-Lys4 position of apamin, comparable low molecular mass (approximately 30 kDa) polypeptides are efficiently labeled on either brain or liver plasma membranes, irrespective of the structure of the photoprobe. However, when photoprobes are coupled at the alpha-amino-Cys1 position of apamin, the pattern of labeling on both brain and liver plasma membranes varies, depending upon the length of the spacer arm incorporated into the photoprobe. Spacer arms of approximately 8-9 A efficiently label only high molecular mass polypeptides (86, 59 kDa), accompanied by weak, variable labeling of a 44-kDa component. A shorter spacer arm (5.7 A) results in feeble labeling of 86- and 59-kDa polypeptides and barely detectable labeling of 44- and approximately 30-kDa polypeptides. In contrast, a long spacer arm (12.8 A) efficiently labels only approximately 30-kDa polypeptides. These findings point to close similarities in the topography of the 125I-apamin binding site present on pharmacologically distinct subtypes of apamin-sensitive Ca2+-activated K+ channels and indicates that heterooligomeric association of high and low molecular mass polypeptide subunits may be a general structural feature of members belonging to this family of K+ channels.
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Affiliation(s)
- J D Wadsworth
- Department of Paediatrics and Neonatal Medicine, Royal Postgraduate Medical School, Hammersmith Hospital, London, U.K
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7
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Abstract
Reabsorption of NaCl in the thick ascending limb of Henle's loop in the kidney and in the surface cells in the distal colon involves the integrated function of several membrane transport systems including ion channels, the Na,K,Cl-cotransport system and the Na,K-pump. To determine if their properties are consistent with a role in regulation of transepithelial transport, Ca(2+)-activated K+ channels from the luminal membrane of the TAL cells and from the basolateral membrane of the distal colon cells have been characterized by flux studies in plasma membrane vesicle preparations and by single channel measurements in lipid bilayers. The channels are found to be activated by Ca2+ in the physiological range of concentration with a strong dependence on intracellular pH and the membrane potential. The Ca(2+)-sensitivity of the K+ channels is modulated by phosphorylation and dephosphorylation and the K+ channel protein must be in a phosphorylated state to respond to intracellular concentrations of Ca2+. As a step towards purification of the K+ channel proteins, procedures for solubilization and reconstitution of the K+ channels have been developed. The observation that the epithelial Ca(2+)-activated K+ channels bind calmodulin in the presence of Ca2+ have allowed for partial purification of the K+ channel proteins by calmodulin affinity chromatography. In the sequences for the two cloned Ca(2+)-activated K+ channels, the mSlo channel and the slowpoke channel, putative calmodulin binding regions can be identified.
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Affiliation(s)
- D A Klaerke
- Biomembrane Research Centre, August Krogh Institute, University of Copenhagen, Denmark
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8
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Wadsworth J, Doorty K, Strong P. Comparable 30-kDa apamin binding polypeptides may fulfill equivalent roles within putative subtypes of small conductance Ca(2+)-activated K+ channels. J Biol Chem 1994. [DOI: 10.1016/s0021-9258(17)32416-x] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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9
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Sokol PT, Hu W, Yi L, Toral J, Chandra M, Ziai MR. Cloning of an apamin binding protein of vascular smooth muscle. JOURNAL OF PROTEIN CHEMISTRY 1994; 13:117-28. [PMID: 7912073 DOI: 10.1007/bf01891999] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
The receptor for the bee venom derived neurotoxin, apamin, is widely believed to be an integral component of the small conductance calcium-activated potassium channel in many excitable cells. By affinity chromatography on immobilized apamin, a 78 kD apamin binding protein of the bovine brain synaptosomes was isolated. Antibodies were elicited against this protein and used to clone a cDNA from a porcine vascular smooth muscle expression library. This gene (Kcal 1.8) codes for a 438 amino protein with four potential transmembrane domains, one putative calcium binding site, a protein kinase C phosphorylation site, and a leucine zipper motif. Kcal 1.8 encoded protein has no significant sequence homologies with any known ion channels or receptors. Kcal 1.8 is likely to encode a protein associated with the small conductance calcium-activated potassium channel in vascular smooth muscle.
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MESH Headings
- Amino Acid Sequence
- Animals
- Antibodies, Monoclonal
- Apamin/metabolism
- Binding Sites
- Blotting, Northern
- Blotting, Western
- Brain/metabolism
- Calcium/metabolism
- Cattle
- Cell Line
- Chromatography, Affinity
- Cloning, Molecular/methods
- Electrophoresis, Polyacrylamide Gel
- Gene Library
- Humans
- Leucine Zippers
- Melanoma
- Molecular Sequence Data
- Molecular Weight
- Muscle, Smooth, Vascular/metabolism
- Neuroblastoma
- Poly A/analysis
- Poly A/biosynthesis
- Potassium Channels/biosynthesis
- Potassium Channels/chemistry
- Potassium Channels/isolation & purification
- Protein Structure, Secondary
- RNA, Messenger/analysis
- RNA, Messenger/biosynthesis
- Recombinant Proteins/biosynthesis
- Recombinant Proteins/chemistry
- Swine
- Synaptosomes/metabolism
- Tumor Cells, Cultured
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Affiliation(s)
- P T Sokol
- Department of Molecular Pharmacology, American Cyanamid Company, Lederle Laboratories, Pearl River, New York 10965
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10
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Shahidi S, Poronnik P, Barden JA, Cook DI. Structure-function relations of biotin derivatives of apamin. BIOCHIMICA ET BIOPHYSICA ACTA 1993; 1157:74-80. [PMID: 8388734 DOI: 10.1016/0304-4165(93)90080-r] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
Apamin was biotinylated at various residues so as to produce an apamin derivative that was suitable for labelling apamin sensitive K+ channels. We labelled the sole histidine residue (His-18) of apamin with diazobenzoyl biocytin (DBB) and Lys-4 with NHS-biotin and NHS-XX-biotin. We found that at least two labelled species were produced by DBB. Proton NMR spectroscopy revealed that in addition to labelling the His-18, DBB labelled the Gln-16 and Gln-17 of apamin. Both NHS-biotin and NHS-XX-biotin appeared to specifically label Lys-4. To test the potency of these apamin derivatives, we developed an assay using apamin reversal of the adrenaline induced relaxation of the mouse ascending colon. The biological activity of the His-18 derivative was 46-fold less than that of native apamin. Biotinylation of Lys-4 with NHS-biotin reduced the activity by only 6-fold. The inclusion of a 14-carbon spacer between the Lys-4 and the biotin resulted in a derivative with only a 4-fold reduction in potency.
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Affiliation(s)
- S Shahidi
- Department of Physiology, University of Sydney, NSW, Australia
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11
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Labbé-Jullié C, Granier C, Albericio F, Defendini ML, Ceard B, Rochat H, Van Rietschoten J. Binding and toxicity of apamin. Characterization of the active site. EUROPEAN JOURNAL OF BIOCHEMISTRY 1991; 196:639-45. [PMID: 2013287 DOI: 10.1111/j.1432-1033.1991.tb15860.x] [Citation(s) in RCA: 50] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
The structural features of apamin, a natural octadecapeptide from bee venom, enabling binding to its receptor and the expression of toxicity in mice, have been delineated by studying the effects on binding and toxicity of chemical modifications and amino acid substitutions in synthetic analogues. The results obtained indicate that the only hydrophobic residue, leucine at position 10, can be changed to alanine without a significant decrease in the specific activity. The need for a correct conformation has been established and also the importance of Gln-17 and the side chains of Arg-13 and Arg-14 (besides the charge effects). The interaction of apamin with its receptor, a calcium-activated potassium channel, is thus mediated by a precise topology around these three residues. Due to the ability to detect very low specific activities for some of the analogues, it has been shown that, individually, none of these interactions constitute an essential criteria for binding per se, but that their presence is necessary for the high specific activity of the toxin.
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Affiliation(s)
- C Labbé-Jullié
- Centre National de la Recherche Scientifique UA 1179, Faculté de Médecine Secteur Nord, Marseille, France
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12
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Leveque C, Marqueze B, Couraud F, Seagar M. Polypeptide components of the apamin receptor associated with a calcium activated potassium channel. FEBS Lett 1990; 275:185-9. [PMID: 2175714 DOI: 10.1016/0014-5793(90)81468-4] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
Photoaffinity labeling of rat brain membranes with [125I]ANPAA-apamin incorporated radioactivity into polypeptides of 86 and 59 kDa and occasionally a more weakly labeled component of 45 kDa. These polypeptides were immunoprecipitated with anti-apamin antibodies and treated with glycosidases. Neither the 86 nor the 59 kDa polypeptide appeared to be N-glycosylated. Partial proteolytic mapping of affinity labeled polypeptides with chymotrypsin or V8 protease generated an identical pattern. These results suggest that the 59 and 45 kDa components are not additional subunits of an oligomeric protein but result from cleavage of the 86 kDa polypeptide.
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Affiliation(s)
- C Leveque
- Laboratoire de Biochimie, CNRS UA1179, Faculté de Médecine Nord, Marseille, France
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13
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14
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Dreyer F. Peptide Toxins and Potassium Channels. Rev Physiol Biochem Pharmacol 1990. [DOI: 10.1007/978-3-662-41884-0_3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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15
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Abstract
Many venom toxins interfere with ion channel function. Toxins, as specific, high affinity ligands, have played an important part in purifying and characterizing many ion channel proteins. Our knowledge of potassium ion channel structure is meager because until recently, no specific potassium channel toxins were known, or identified as such. This review summarizes the sudden explosion of research on potassium channel toxins that has occurred in recent years. Toxins are discussed in terms of their structure, physiological and pharmacological properties, and the characterization of toxin binding sites on different subtypes of potassium ion channels.
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Affiliation(s)
- P N Strong
- Jerry Lewis Muscle Research Centre, Department of Paediatrics and Neonatal Medicine, Royal Postgraduate Medical School, London, U.K
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16
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Lambeau G, Barhanin J, Schweitz H, Qar J, Lazdunski M. Identification and Properties of Very High Affinity Brain Membrane-binding Sites for a Neurotoxic Phospholipase from the Taipan Venom. J Biol Chem 1989. [DOI: 10.1016/s0021-9258(18)60492-2] [Citation(s) in RCA: 139] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022] Open
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17
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Marquèze B, Seagar MJ, Couraud F. Characterization of apamin-binding protein associated with a Ca2+ -activated K+ channel. JOURNAL OF PROTEIN CHEMISTRY 1989; 8:425-7. [PMID: 2551328 DOI: 10.1007/bf01674309] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Affiliation(s)
- B Marquèze
- Zentrum für Molekulare Biologie, Universität Heidelberg, FRG
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18
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Klaerke DA, Jørgensen PL. Role of Ca2+-activated K+ channel in regulation of NaCl reabsorption in thick ascending limb of Henle's loop. COMPARATIVE BIOCHEMISTRY AND PHYSIOLOGY. A, COMPARATIVE PHYSIOLOGY 1988; 90:757-65. [PMID: 2902984 DOI: 10.1016/0300-9629(88)90695-0] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
1. Reabsorption of NaCl in the thick ascending limb of Henle's loop involves the integrated function of the Na+,K+,Cl- -cotransport system and a Ca2+-activated K+ channel in the luminal membrane with the Na+,K+-pump and a net Cl- conductance in the basolateral membrane. 2. Assay of K+ channel activity after reconstitution into phospholipid vesicles shows that the K+ channel is stimulated by Ca2+ in physiological concentrations and that its activity is regulated by calmodulin and phosphorylation from cAMP dependent protein kinase. 3. For purification luminal plasma membrane vesicles are isolated and solubilized in CHAPS. K+ channel protein is isolated by affinity chromatography on calmodulin columns. The purified protein has high Ca2+-activated K+ channel activity after reconstitution into vesicles. 4. The purified K+ channel consists of two proteins of 51 and 36 kDa. Phosphorylation from cAMP dependent protein kinase stimulates K+ channel activity and labels the 51 kDa band. The 36 kDa band is rapidly cleaved by trypsin and may be involved in Ca2+ stimulation. 5. Opening of the K+ channel by Ca2+ in physiological concentrations and regulation by calmodulin and phosphorylation by protein kinase may mediate kinetic and hormonal regulation of NaCl transport across the tubule cells in TAL.
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Affiliation(s)
- D A Klaerke
- Institute of Physiology, Aarhus University, Denmark
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19
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Marquèze B, Seagar MJ, Couraud F. Photoaffinity labeling of the K+-channel-associated apamin-binding molecule in smooth muscle, liver and heart membranes. EUROPEAN JOURNAL OF BIOCHEMISTRY 1987; 169:295-8. [PMID: 2446869 DOI: 10.1111/j.1432-1033.1987.tb13611.x] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
High-affinity binding sites for mono[125I]iodoapamin were detected in membranes (Kd = 59 pM, Bmax = 24 fmol/mg protein) and cultured cells (Kd = 69 pM, Bmax = 2.8 fmol/mg protein) from rat heart and in membranes from guinea-pig ileum (Kd = 67 pM, Bmax 42 fmol/mg protein) and liver (Kd = 15 pM, Bmax = 43 fmol/mg protein). Binding was stimulated by K+ ions (K0.5 = 0.3-0.5 mM). Covalent labeling with arylazide [125I]iodoapamin derivatives showed that smooth muscle, liver and heart binding molecules are associated with a 85-87-kDa polypeptide. A second strongly labeled 57-kDa component was identified in liver membranes only.
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Affiliation(s)
- B Marquèze
- Laboratoire de Biochimie, Centre National de la Recherche Scientifique Unité Associée 1179, Marseille-France
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20
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Klaerke DA, Petersen J, Jørgensen PL. Purification of Ca2+-activated K+ channel protein on calmodulin affinity columns after detergent solubilization of luminal membranes from outer renal medulla. FEBS Lett 1987; 216:211-6. [PMID: 2438163 DOI: 10.1016/0014-5793(87)80691-9] [Citation(s) in RCA: 28] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
A method is developed for purification of the protein of the Ca2+-activated K+ channel from outer renal medulla of pig kidney. The response of this K+ channel to physiological concentrations of Ca2+ is important for regulation of transtubular NaCl transport. In reconstituted vesicles direct addition of calmodulin doubles Ca2+ activation with sufficient affinity (K1/2 0.1 nM) for chromatographic purification of the protein. For purification luminal plasma membrane vesicles are isolated on metrizamide density gradients and solubilized in CHAPS. The fraction of soluble protein retained on calmodulin-Sepharose 4B columns in the presence of Ca2+ and eluted by EGTA is 0.7%. The purified protein has high Ca2+-activated K+ channel activity after reconstitution into phospholipid vesicles. It distributes on two bands of 51 and 36 kDa after gel electrophoresis in SDS. The 36 kDa band is rapidly cleaved by trypsin and may be involved in Ca2+ stimulation of the channel. Phosphorylation from cAMP-dependent protein kinase strongly stimulates Ca2+-activated K+ channel activity and labels the 51 kDa band suggesting that this protein is involved in regulation of K+ channel opening.
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21
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Seagar MJ, Deprez P, Martin-Moutot N, Couraud F. Detection and photoaffinity labeling of the Ca2+-activated K+ channel-associated apamin receptor in cultured astrocytes from rat brain. Brain Res 1987; 411:226-30. [PMID: 2440516 DOI: 10.1016/0006-8993(87)91073-0] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
Apamin, an 18-amino acid bee venom peptide, is a specific blocker of a class of Ca2+ activated K+ channels. Mono 125I-iodoapamin was used to detect the K+ channel-associated receptor site in cultured astrocytes from rat brain. Specific high-affinity binding to intact glial cells with a Kd of about 90 pM at 1 degree C and pH 7.5 was demonstrated by equilibrium and kinetic methods. The average receptor capacity was 3 fmol/mg cell protein which is 2 to 3-fold lower than in primary cultured neurons. Binding was stimulated by K+ ions, but to a lesser extent than with neuronal receptors. Photoaffinity labeling of receptor/ion channel components using an arylazide derivative of 125I-monoiodoapamin revealed the presence of the 86- and 33-kDa polypeptides, previously detected in neurones. However a 59-kDa peptide which is present in synaptic membrane preparations from adult rat brain, but not in cultured neurons, was also clearly labeled in intact astrocytes. This indicates that the 59-kDa polypeptide is not a proteolytic fragment of the 86-kDa chain but an associated subunit which is only accessible to photolabeling in certain apamin receptor preparations. Apamin-sensitive Ca2+-activated K+ channels in astrocytes may be one of the pathways by which glial cells redistribute K+ in the central nervous system (CNS).
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22
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Seagar MJ, Labbé-Jullié C, Granier C, Goll A, Glossmann H, Van Rietschoten J, Couraud F. Molecular structure of rat brain apamin receptor: differential photoaffinity labeling of putative K+ channel subunits and target size analysis. Biochemistry 1986; 25:4051-7. [PMID: 2427110 DOI: 10.1021/bi00362a010] [Citation(s) in RCA: 46] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
Two photoreactive apamin derivatives were prepared with an aryl azide [[(azidonitrophenyl)amino]acetate (ANPAA)] group coupled at different positions on the neurotoxin molecule. These ligands were used to identify membrane components in the environment of the neuronal binding site that is associated with a Ca2+-activated K+ channel. 125I-[alpha-ANPAA-Cys1] apamin labeled a single Mr 86 000 chain in cultured neurons whereas two bands corresponding to Mr 86 000 and 59,000 were detected in synaptic membrane preparations, suggesting that the Mr 59,000 polypeptide may be a degradation product. 125I-[epsilon-ANPAA-Lys4]apamin however incorporated uniquely into two smaller components with Mr 33,000 and 22,000 in both cultured neurons and synaptic membranes. Randomly modified 125I-ANPAA-apamin gave a cross-linking profile equivalent to the sum of those obtained with the two defined derivatives. The apamin binding site seems to be located at the frontier between three or more putative K+ channel subunits which are only accessible from limited regions of the receptor-associated photoprobe. Irradiation of frozen rat brain membranes with high-energy electrons led to a reduction in 125I-apamin receptor capacity, yielding a target size for the functional binding unit of Mr 84,000-115,000, which could be constituted by the Mr 86,000 subunit alone or by the Mr 86,000 subunit in conjuction with one of the two smaller subunits.
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23
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